The polycarbonate of 4,4'-isopropylidenediphenol (bisphenol A polycarbonate) is a well known engineering molding plastic. Bisphenol A polycarbonate is a clear high-performance plastic having good physical properties such as dimensional stability, high heat resistance, and good impact strength. Although bisphenol A polycarbonate has many good physical properties, its relatively high melt viscosity leads to poor melt processability and the polycarbonate exhibits poor chemical resistance.
Blends display different physical properties based upon the nature of the polymers blended together as well as the concentration of each polymer in the blend. Attempts have been made to blend bisphenol A polycarbonate with other polymers that have good chemical resistance, processability, and machinability. These attempts to improve melt processability, chemical resistance and other physical properties of bisphenol A polycarbonate have been made by blending bisphenol A polycarbonate with polymers such as polystyrene, elastomers, polyesters, and polyesterimides. However, blends of bisphenol A polycarbonate with other polymeric materials have usually resulted in immiscible blend compositions. Immiscible blend compositions are inadequate for many uses because they are not clear.
Clear, miscible blends of any two polymers are rare. The term "miscible" refers to blends that are a mixture on a molecular level wherein intimate polymer-polymer interaction is achieved. Miscible blends are clear, not translucent or opaque. In addition, differential scanning calorimetry testing detects only a single glass transition temperature (Tg) for miscible blends composed of two or more components.
There have been very few clear polycarbonate/polyester blends developed. U.S. Pat. Nos. 4,619,976 and 4,645,802 disclose clear blends based on bisphenol A polycarbonate with polyesters of poly(1,4-tetramethylene terephthalate), poly(1,4-cyclohexylenedimethylene terephthalate) and selected copolyesters and copoly(esterimides) of poly(1,4-cyclohexylenedimethylene terephthalate). U.S. Pat. No. 4,786,692 discloses clear blends of bisphenol A polycarbonate and polyesters of terephthalic acid, isophthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol. U.S. Pat. Nos. 4,188,314 and 4,391,954 disclose clear blends of bisphenol A polycarbonate with poly(l,4-cyclohexylenedimethylene terephthalate-co-isophthalate). These polyester blends do have improved chemical resistance and melt processability, when compared to unblended bisphenol A polycarbonate. However, the good heat resistance and impact strength of bisphenol A polycarbonate blends based on these compositions is reduced significantly.
In light of the above it would be very desirable to have a new clear polymer blend based upon bisphenol A polycarbonate having improved melt processability and good chemical resistance, while retaining good heat resistance and excellent impact strength. Such blend would be especially suitable for the manufacture of clear molded articles, fibers, sheeting, and film.